Motor control device and motor control method

ABSTRACT

A motor control device includes a detecting unit configured to detect rotation of a motor to be controlled and output a rotation detection value related to the rotation; a drive control unit configured to perform drive control to rotate the motor at a control target value increasing with time based on the rotation detection value; and an abnormality detection unit configured to perform an abnormality detection process for detecting an abnormality in the drive control based on the rotation detection value and a predetermined threshold. The drive control unit performs control to stop rotation of the motor when the abnormality is detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/755,353 filed on Jan. 31, 2013, now issued as U.S. Pat. No.9,484,845, which claims priority to and incorporates by reference theentire contents of Japanese Patent Application No. 2012-018730 filed inJapan on Jan. 31, 2012 and Japanese Patent Application No. 2012-277471filed in Japan on Dec. 19, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor control device, a motor controlmethod, and a computer-readable storage medium.

2. Description of the Related Art

In various types of electric equipment, a position control system or aspeed control system, in which a direct-current motor (referred to as a“DC motor” herein after) is used as a driving source, has beenconventionally used in an operation unit. For example, in an imageformation device such as a printer and an multifunction machine, acontrol system for feeding back position information and speedinformation generated from a rotation detection signal (encoder signal)of a motor is used for a drive control of a DC motor used for conveyinga paper such as a recording paper and a document to be copied.

As a conventional technique related to the drive control of theabove-described DC motor in an image formation device, a technique ofJapanese Patent No. 2886534, for example, is known. Japanese Patent No.2886534 discloses that an abnormality of rotation is detected from anoutput of an encoder and when an abnormality is detected, a process ofturning off a driver of a motor is performed in order to prevent heatingor ignition of a motor caused by an abnormality operation, which mayoccur in a feedback control system for controlling the driving of aservo (DC) motor, from occurring.

However, this type of conventional technique discloses a configurationapplicable only for a limited condition that an abnormality detection isperformed using an output of an encoder that detects rotation of a motorduring phase-locked loop (PLL) control for an operation at a constantspeed. Therefore, when a DC motor operates to monotonically increase itsrotation speed with time such as an acceleration operation from astopped state, it is difficult to detect an occurrence of an abnormalitywhich may be a situation that a rotation detection signal is notnormally output from an encoder because of a disconnection or a circuitfailure, for example. Thus, a breakage of a mechanical system due to itsrunaway, damage due to motor heating, and the like is unfortunatelycaused.

Therefore, there is a need to provide a motor control device, a motorcontrol method, and a computer-readable storage medium capable ofpreventing a damage of a motor driving system due to an abnormality bydetecting an abnormality occurring upon a drive control for increasingrotation of a motor with time.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

According to an embodiment, there is provided a motor control devicethat includes a detecting unit configured to detect rotation of a motorto be controlled and output a rotation detection value related to therotation; a drive control unit configured to perform drive control torotate the motor at a control target value increasing with time based onthe rotation detection value; and an abnormality detection unitconfigured to perform an abnormality detection process for detecting anabnormality in the drive control based on the rotation detection valueand a predetermined threshold. The drive control unit performs controlto stop rotation of the motor when the abnormality is detected.

According to another embodiment, there is provided a motor controlmethod that includes detecting rotation of a motor to be controlled;outputting a rotation detection value related to the rotation;performing drive control to rotate the motor at a control target valueincreasing with time based on the rotation detection value; performingan abnormality detection process for detecting an abnormality in thedrive control based on the rotation detection value and a predeterminedthreshold; and performing control to stop rotation of the motor when theabnormality is detected.

According to still another embodiment, there is provided anon-transitory computer-readable storage medium with an executableprogram stored thereon. The program instructs a computer to perform:detecting rotation of a motor to be controlled; outputting a rotationdetection value related to the rotation; performing drive control torotate the motor at a control target value increasing with time based onthe rotation detection value; performing an abnormality detectionprocess for detecting an abnormality in the drive control based on therotation detection value and a predetermined threshold; and performingcontrol to stop rotation of the motor when the abnormality is detected.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a motor controldevice according to an embodiment;

FIG. 2 is a motor feedback control system in the motor control deviceaccording to the embodiment in functional blocks;

FIG. 3 is a graph illustrating a control target position that is afunction of time and detected positions of an encoder both in abnormalconditions and in normal conditions in contrast;

FIG. 4 is a graph to explain a dead time in a control operation by themotor control device according to the embodiment;

FIG. 5 is a flow chart illustrating a procedure of an abnormalitydetection process (detection method 2) according to the embodiment;

FIG. 6 is a graph illustrating the control target position that is afunction of time, and position errors both in abnormal conditions and innormal conditions in contrast; and

FIG. 7 is a flow chart illustrating a procedure of an abnormalitydetection process (detection method 1) according to the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the accompanying drawings, embodiments according tothe present invention will be described in detail below. In theembodiments, a motor control device that uses a direct-current motor(referred to as “DC motor”, hereinafter) as a driving source for anoperation of conveying a moving body to a predetermined position at apredetermined speed will be described as an example.

More specifically, the motor control device according to the embodimentsperforms a control of: feeding back a detected speed and a detectedposition, which can be obtained from a rotation detection signal of anencoder attached to the output shaft of the motor in a state that a loadfor conveying the moving body is applied to the DC motor, to acontroller for controlling the driving of the motor; conveying themoving body at a speed of control target; and positioning the movingbody, as an example. Note that the motor control device may beconfigured to have a control function only for speed control toaccelerate a moving body from a stopped state to a given speed. Themotor control device may also be configured to have a control functiononly for position control under operation condition of monotonicallyincreasing the speed. “Monotonically increasing the speed” herein meansincreasing the speed without up-and-down variation.

Configuration of Motor Control Device

FIG. 1 is a diagram illustrating a configuration of a motor controldevice according to an embodiment. The motor control device illustratedin FIG. 1 is an example in a case where a DC motor is used as a drivingsource of a device for conveying a paper medium such as a recordingpaper and a document handled in equipment such as a copying machine, ascanner device, a facsimile device, and a printer device. Arrows betweencomponents in FIG. 1 indicate information (data) flow or an operationalrelationship (relationship of causal connection).

The motor control device of this embodiment essentially includes: atarget position setting unit 8; a target speed setting unit 1; acontroller 2; a PWM converting unit 3; a motor driver 4; a DC motor+load5; an encoder 6; and a speed detecting unit 7; and a position detectingunit 9 as illustrated in FIG. 1.

The motor control device controls to convey a paper medium at a targetspeed as a control target and to position the paper medium using the DCmotor as a driving source. Therefore, the motor control device controlsthe motor driver 4 that drives the DC motor as a control target in asystem including: the controller 2, the PWM converting unit 3; thetarget speed setting unit 1, and the target position setting unit 8. Thetarget speed setting unit 1 sets the target speed as a control targetvalue to the controller 2 by a user input operation before controloperation. The target position setting unit 8 sets a target position asa control target value to the controller 2 by a user input operationbefore control operation.

In addition, the motor control device forms a feedback loop to thecontroller 2, the feedback loop including: the encoder 6; the speeddetecting unit 7; and the position detecting unit 9. The encoder 6 isattached to the rotating shaft of the DC motor, and is configured todetect rotation. As the encoder 6, an encoder that detects light actingslits arranged at equal intervals in a circumferential direction of adisc using a sensor and converts rotation to an encoder signal includinga time series pulse train (the rotation speed is detected as a pulsefrequency) may be used, for example.

Note that the DC motor drives a means for conveying a paper medium suchas a recording paper and a document, and thus an operating state uponconveying operation under load, i.e. the movement of the rotation(output) shaft of the “DC motor+load 5” as illustrated in FIG. 2 is thetarget to be detected by the encoder 6 and output as an encoder(rotation detection) signal.

The speed detecting unit 7 detects the rotation speed of the DC motorfrom the encoder signal output from the encoder 6. In addition, theposition detecting unit 9 detects the rotation position of the DC motorfrom the encoder signal output from the encoder 6. Each of the rotationspeed and the rotation position detected here corresponds to a rotationdetection value. The rotation speed detected by the speed detecting unit7 is hereinafter referred to as a detected speed, and the rotationposition detected by the position detecting unit 9 is hereinafterreferred to as a detected position.

The controller 2 includes a control unit 2 e and an abnormalitydetection unit 2 d as illustrated in FIG. 1. The control unit 2 e servesas a drive control unit, and performs a drive control to rotate the DCmotor+load 5 at a control target value increasing with time based on thedetected speed from the speed detecting unit 7, and the detectedposition from the position detecting unit 9. Specifically, the controlunit 2 e uses a difference of the detected speed of the DC motorobtained from a difference between the target speed set by the targetspeed setting unit 1 and the detected speed detected by the speeddetecting unit 7 coupled with a difference between the target positionset by the target position setting unit 8 and the detected positiondetected by the position detecting unit 9. From the difference of thedetected speed, the control unit 2 e performs PID (P: proportion, I:Integration, D: Differentiation) calculation, low pass filtercalculation, and the like so as to calculate a control amount. Thedetail of this control unit 2 e will be described later using FIG. 2.

In addition, the controller 2 includes the abnormality detection unit 2d. The abnormality detection unit 2 d obtains an error of the detectionresults of the speed detecting unit 7 and the position detecting unit 9with respect to the control target values, and from the obtained errors,it detects an abnormality of the control operation. Alternatively, theabnormality detection unit 2 d may detect an abnormality of the controloperation position using the detected position from the positiondetecting unit 9. The control unit 2 e of the controller 2 controls tostop driving of the DC motor if the abnormality detection unit 2 ddetects an abnormality of the control operation. The detail of thisabnormality detection process will be described later.

The PWM converting unit 3 receives input of digital data representing acontrol amount calculated by the controller 2, and converts the digitaldata to a PWM (Pulse Width Modulation) signal having a duty proportionalto the control amount. The motor driver 4 drives the DC motor byapplying to the DC motor a voltage proportional to the duty of the PWMsignal output from the PWM converting unit 3.

The DC motor is controlled by the motor control device including thecomponents illustrated in FIG. 1 to rotate at the target speed set bythe target speed setting unit 1, whereby driving the means for conveyinga paper medium (not shown) such as a recording paper and a document soas to move the paper medium to the target position set by the targetposition setting unit 8.

An additional description of the feedback control system of the motorcontrol device (FIG. 1) is now provided. FIG. 2 is a diagramillustrating the DC motor feedback control system in the motor controldevice of FIG. 1 in functional blocks. FIG. 2 is a diagram expressingthe functions of the respective components as their transfercharacteristics in association with the components illustrated inFIG. 1. Note that, FIG. 2 illustrates, as a basic configuration of thecontrol system, a configuration during normal operation in which the DCmotor is controlled according to the set condition by the feedbackcontrol function. In FIG. 2, blocks in the controller 2 represent aconfiguration of the control unit 2 e.

Since the control unit 2 e of the controller 2 performs the feedbackcontrol using PID operation, it performs a PID calculation, a low passfilter calculation, and the like. Therefore, as illustrated in FIG. 2,the control unit 2 e of the controller 2 calculates a difference betweenthe target position set by the target position setting unit 8 and therotation position (detected position by a Gp module 91 of the positiondetecting unit 9) of the DC motor as a control target, multiplies thedifference by a Gx (position feedback gain) at a Gx module 21, and addsthe multiplication result to a difference between the target speed setby the target speed setting unit 1 and the rotation speed (detectedspeed by a Gv module 71 of the speed detecting unit 7) of the DC motoras a control target.

The control unit 2 e of the controller 2 multiplies the calculateddifference by P (proportional gain) at a P module 22, multiplies theresultant by I (Integration gain)/s (Laplace operator) at an I/s module23 and by D (Differentiation gain)*s at a D*s module 24 respectively,and adds respective calculation results of the P module 22, the I/smodule 23, and the D*s module 24, as PID calculation.

In addition, the control unit 2 e of the controller 2 multiplies thecalculation result of the PID calculation by LPF(s) (low pass filter) atan LPF(s) module 25 so as to obtain a control amount.

The PWM converting unit 3 multiplies the control amount [lsb], which hasbeen calculated by the controller 2, by a Gpwm [%/lsb] gain forconverting the control amount to a duty [%] of the PWM signal at a Gpwmmodule 31. The motor driver 4 multiplies the duty of the PWM signaloutput from the PWM converting unit 3 by a Gd [V/%] gain for convertingthe duty to a voltage Vm to be applied to the DC motor at a Gd module41.

The DC motor+load 5, to which the voltage Vm is applied from the motordriver 4, can be represented as a system having a module configurationincluding: a 1/(R+L*s) module 51; a Kt module 52; addition of anabnormal disturbance torque; torque variation; a 1/J module 53; and a1/s module 54 in order from the voltage Vm input side to the rotationoutput Vw side, and further including a Ke module 55 on a loop forfeeding back the rotation output Vω. The parameters of the DC motor+load5 in the above module configuration are provided below.

R: Inter-terminal resistance

L: motor coil inductance

Kt: torque constant

J: motor inertia+load inertia

s: Laplace operator

Ke: inductive voltage constant

The encoder 6, which receives the rotation output Vo of the DC motor andoutputs the encoder (rotation detection) signal, calculates a Ge[Hz/(rad/s)] gain for converting a pulse number generated while therotating shaft of the DC motor rotates one revolution to a valueobtained by dividing the pulse number by 2π at a Ge module 61.

When a method of detecting a frequency by counting a pulse period of anencoder pulse by a high-speed clock fc [Hz] is used at the Gv module 71,for example, an encoder pulse frequency can be calculated by dividingthe high-speed clock fc [Hz] by the count value. Therefore, the speeddetecting unit 7, which receives the rotation detection signal from theencoder 6, converts the pulse period of the encoder pulse to a detectedspeed by calculation using a Gv [lsb/Hz] gain at a Gv module 71.

The position detecting unit 9, which receives the rotation detectionsignal from the encoder 6, converts the rotation detection signal to therotation position (detected position) by multiplying the count value ofthe encoder pulse by a Gp [lsb·rad] gain at the Gp module 91.

A computer used as the controller 2, which is not illustrated, may beconfigured with a general-purpose hardware. More specifically, in thecomputer, a central processing unit (CPU) for executing instructions insoftware programs; a read only memory (ROM) for storing programs, data,and the like used by the CPU; a random access memory (RAM) fortemporarily storing data generated by the programs and the like; and ahard disk drive (HDD) for storing data required to be stored such asvarious setting data set to the controller 2 and management informationin a non-volatile manner are respectively connected by buses, and thebuses have interfaces for respectively connecting an I/O port, a displayunit, and an input unit.

The I/O port of the computer, which is used, is used as an input portfor detected speed data calculated at the speed detecting unit 7 basedon the rotation detection signal from the encoder 6 and detectedposition data calculated at the position detecting unit 9. The displayunit and the input unit of this computer display a screen for guidinginput operation of a user, and receive input of instructions and dataregarding to settings made by button (key) operations of a userperformed using a mouse and the like following the guide displayed onthe screen, as the target position setting unit 8 and the target speedsetting unit 1.

By connecting a device for reading/writing various disc-shaped mediasuch as a compact disc (CD) or a small flash memory to the I/O port,various removable recording media can be used.

Abnormality Detection Process

If an abnormality that the rotation detection signal is not output fromthe encoder 6 due to disconnection, for example occurs during control ofincreasing the rotation speed of the DC motor with time such asacceleration from a stopped state by the motor control device describedreferring to FIGS. 1 and 2, the controller 2 controls in such a mannerthat the DC motor is at a predetermined target speed, that is, thedifference is eliminated. However, since the target speed increasesmonotonically in an acceleration operation, the difference may increaseso as to continuously increase current flowing into the DC motor andthen runaway of the DC motor is caused. Thus, there is a possibility ofbreakage of parts due to runaway of a mechanical system that is driven,and of damage due to motor heating. Such an accident may also occur in acase where the rotation detection signal is not normally output from theencoder 6 due to a circuit malfunction and the like other thandisconnection.

Therefore, the motor control device detects an abnormality occurring inthe rotation detection signal from the encoder 6, and performs a stopcontrol on the DC motor as a control target when an abnormality isdetected, thereby preventing a damage and the like due to an abnormaloperation of the DC motor from occurring.

An abnormality occurring in the rotation detection signal may bedetected by either of the following two detection methods. In anacceleration operation of the DC motor as a control target, the controltarget value monotonically increases with respect to the elapsed time astime passes. Therefore, in a first abnormality detection method, athreshold processing for comparing the rotation detection value obtainedbased on the rotation detection (encoder) signal of the DC motor at thetime when a predetermined time period has elapsed with a predeterminedfirst threshold is performed, and an abnormality is detected accordingto this comparison result (referred to as “detection method 1”hereinafter). In a normal operation, the rotation detection value shouldincrease. Therefore, for this detection method 1, the first threshold isset to be a value that is supposed to be exceeded by the rotationdetection value at the time when the predetermined time period haselapsed, and if the rotation detection value at the time when thepredetermined time period has elapsed is smaller than or equal to thisfirst threshold, it is determined that an abnormality occurs and thedetermination is set as a detection result.

In an acceleration operation of the DC motor as a control target, thecontrol target value monotonically increases with respect to the elapsedtime as time passes (i.e. can be determined uniquely). Therefore, in asecond abnormality detection method, an error of the rotation detectionvalue obtained based on the rotation detection (encoder) signal of theDC motor at the time when a predetermined time period has elapsed withrespect to the control target value at the time (corresponding to adifference of the feedback control) is calculated, threshold processingfor comparing the calculated error with a predetermined second thresholdis performed, and an abnormality is detected based on this comparisonresult (referred to as “detection method 2” hereinafter). In a normaloperation, the rotation detection value should increase and the errorwith respect to the control target value should fall in a small value.Therefore, for this detection method 2, the second threshold is set tobe a value that is not supposed to be exceeded by the error, and if theerror at the time when the predetermined time period has elapsed exceedsthis second threshold, it is determined that an abnormality occurs andthe determination is set as a detection result (refer to the descriptionof FIG. 5 below).

These abnormality detection methods are described in more detail withconcrete examples. FIG. 3 is a graph illustrating a control targetposition that is a function of time and detected positions of theencoder both in abnormal conditions and in normal conditions incontrast. In FIG. 3, the longitudinal axis represents a rotationposition of the DC motor and defined by a detected position [puls]obtained based on a rotation detection signal from the encoder 6 (notethat the line Lg illustrating a target position does not represent adetected value), and the horizontal axis represents time [t] that is anelapsed time from the time point when the control operation is started,which is set as t=0.

FIG. 3 illustrates detection values as time passes from the controlstart time both in abnormal conditions and in normal conditions in acontrol operation of simply increasing a target position as a controltarget value (illustrated example is a constant acceleration operation).

The motor control device is intended to perform detection operations forabnormalities of different causes. In FIG. 3, respective detectionvalues are illustrated as an abnormality 1 (first abnormality) and anabnormality 2 (second abnormality).

The abnormality 1 is a case where a rotation detection signal is notoutput from the encoder 6 due to disconnection, for example. On theother hand, the abnormality 2 is a case where a rotation detectionsignal indicating a direction opposite to the actual rotation is outputfrom the encoder 6 due to a malfunction of the circuit, for example. Inorder to enable detection of rotation direction, a rotation detection(encoder) signal of two phases shifted by 90° (phase A and phase B) isoutput in this embodiment. If the phase A and the phase B are connectedoppositely by a malfunction, for example, the rotation direction isdetermined to be opposite, and positive and negative of the displacementalso become opposite, and thus it is unfortunately detected as anoperation moving away from the target position.

In the graph of FIG. 3, in conditions that a rotation detection signalis normally output from the encoder 6, the control is performed in sucha manner that the detected position detected by the position detectingunit 9 becomes the same as the target position once the DC motor controlis started. Therefore, a line Ln representing a detected position innormal conditions represents a position slightly lower than but close toa line Lg representing a target position in FIG. 3.

On the other hand, in conditions of an abnormality 1, if a rotationdetection (encoder) signal is not output due to disconnection, thedetected position stays at zero and does not vary even if the DC motorrotates as represented by a line L1 representing a detected position inthe conditions of the abnormality 1 in FIG. 3.

In the conditions of the abnormality 2, if a rotation detection(encoder) signal is of incorrect opposite rotation output, a detectedposition output from the position detecting unit 9 based on theincorrect opposite rotation output becomes a negative value asrepresented by a line L2 representing a detected position in theconditions of the abnormality 2 in FIG. 3.

As described above, in the conditions of the abnormality 1, as time [t]passes, a difference between the target position Lg and the line L1representing a detected position in the conditions of the abnormality 1illustrated in FIG. 3 increases as the target position Lg increases.Therefore, the controller 2 performs a control to rotate the DC motor ata high speed in order to quickly eliminate the difference.

In the conditions of the abnormality 2, a difference between the targetposition Lg and the line L2 representing a detected position in theconditions of the abnormality 2 illustrated in FIG. 3 rapidly increaseswhen a position variation in the negative direction due to falsedetection represented by the line L2 is added to the increase of thetarget position Lg as time [t] passes. Therefore, the controller 2performs a control to rotate the DC motor at a higher speed than in theconditions of the abnormality 1 in order to eliminate the rapidlyincreasing difference.

Which means that the motor control device performs an unintended controloperation corresponding to a difference that is generated because ofoutput of an incorrect rotation detection signal from the encoder 6 andthat is impossible in normal conditions in the both conditions of theabnormality 1 and the abnormality 2, and the unintended controloperation damages a DC motor driving system.

In order to prevent the above-described unintended control operationcorresponding to the output abnormality of a rotation detection signalfrom occurring, the motor control device detects an abnormality by theabove-described detection method 1 or 2 based on a rotation detection(encoder) signal of the DC motor at the time when a predetermined timeperiod has elapsed in the acceleration operation.

More specifically, in the case of the detection method 1, theabnormality detection unit 2 d detects occurrence of an abnormality witha condition that the rotation detection value is smaller than or equalto the first threshold for determining whether it is a normal value ornot. In the case of the detection method 2, the abnormality detectionunit 2 d calculates an error of the rotation detection value withrespect to the control target value, and detects occurrence of anabnormality with a condition that the calculated error is more than thesecond threshold for determining whether it is a value corresponding tooccurrence of an abnormality.

As the first and second thresholds used to detect an abnormality,experiential values obtained by verifying conditions that safety can besecured in the driving system as a control target, and that an effect ofthe abnormality detection process on the primary motor control operationcan be small are used.

The above-described abnormality detections corresponding to theabnormality 1 and the abnormality 2 are both performed by thresholdprocessing of the position detection value obtained based on therotation detection (encoder) signal or an error of the positiondetection value with respect to the control target value. Therefore, theabnormality detection unit 2 d may be configured to perform abnormalitydetection each time when a rotation detection signal is sampled.

It should be noted that there are time periods when a detection resultis not effectively used, or there is no point in obtaining a detectionresult. These time periods include: a time period corresponding to adead time that is a certain time period from a start time of theacceleration control; and a time period which is after a normaloperation is confirmed by an abnormality detection and thus in which itis thought that there is no point in obtaining a detection result withina time period for an acceleration control. The time periods aredetermined by points indicated as “detection start position” and“detection end position”, respectively, in FIG. 3. A time periodcorresponding to the positions between the two points is a time periodto perform an abnormality detection process, that is, a detection timeperiod when abnormality detection is required.

In the motor control device, this time period when abnormality detectionis required is applied only to detection of the abnormality 1, anddetection function is basically always operated for the abnormality 2.This is because of the nature that the detection value varies less inabnormal conditions of the abnormality 1 (the detection value stays atzero due to disconnection).

FIG. 4 is a graph to explain the dead time in the control operation bythe motor control device. In FIG. 4, the longitudinal axis represents arotation position of the DC motor (note that the line Lg represents theset target position and the line Ln represents a detected position), andthe horizontal axis represents time [t] that is an elapsed time from thetime point when the control operation is started, which is set as t=0,similarly to FIG. 3.

A control operation of simply increasing a target position as a controltarget value as time passes from the control start time as illustratedas a line Lg (illustrated example in FIG. 4 is a constant accelerationoperation) is operated, and in this control operation, a detectedposition when a normal rotation detection (encoder) signal is outputrepresented by a line Ln is illustrated in FIG. 4.

The time period from the start time of the control operation to a timepoint t0 in FIG. 4 is immediately after the start of the accelerationoperation of the DC motor. In an operating state affected by staticfriction torque and the like in this time period, an output of adetected position from the position detecting unit 9 is delayed by atime length t0 from the operation start time. This time lengthcorresponding to t0 is called dead time.

Therefore, an abnormality detection based on a detected position outputfrom the position detecting unit 9 in this dead time t0 may lead to anincorrect detection result, and thus the detection result is not used.Therefore, the abnormality detection unit 2 d starts abnormalitydetection after the dead time t0, which is a predetermined time, haselapsed.

The “detection end position” indicated in FIG. 3 defines a time pointwhen a detected position output from the position detecting unit 9 isconfirmed to be normal, and thus in a time period after the time point,it is much less likely that a detection result changes to abnormal.Therefore, it is beneficial to finish the process so as to reduce theprocess load. This operation is especially well-suited for detection ofthe abnormality 1, and is used in an abnormality detection process to bedescribed later (refer to the flow of FIG. 5).

Abnormality Detection Process Flow

A procedure in the abnormality detection process performed by thecontroller 2 (abnormality detection unit 2 d) of the motor controldevice will be described referring to a process flow chart of FIG. 5. Inthe flow chart of the abnormality detection process of FIG. 5, theabnormality detection unit 2 d performs the detection process adapted toboth of the different causes of abnormality including: the abnormality 1(a case where a rotation detection signal is not output from the encoder6 due to disconnection, for example); and the abnormality 2 (a casewhere a rotation detection signal in a direction opposite to the actualrotation is output from the encoder 6 due to a malfunction of thecircuit, for example). In the flow chart illustrated in FIG. 5, theabnormality detection unit 2 d detects an abnormality by theabove-described detection method 2 (that is a method including:calculating an error of the rotation detection value with respect to thecontrol target value; and detecting occurrence of an abnormality with acondition that the calculated error is more than the second thresholdfor determining whether it is a value corresponding to occurrence of anabnormality).

When the motor control device controls the DC motor in a stopped state,for example, as a control target, the abnormality detection unit 2 dstarts by activating the process flow chart of FIG. 5.

When the process according to the flow chart of FIG. 5 is started, theabnormality detection unit 2 d firstly confirms that the started controlthat is started when the drive control of the DC motor is started is anacceleration operation if the target position set as the control targetvalue increases (step S101: Yes). Note that although the abnormalitydetection unit 2 d herein confirms that the acceleration operation isstarted from the stopped state based on the increase of the targetposition as time passes, the abnormality detection unit 2 d may beconfigured to determine speed variation from the stopped state (or aconstant-speed operating state) based on the target speed. In addition,if a high level controller for controlling the motor control deviceexists, and the operation is started according to a stop request,acceleration request, and the like from the high level controller, theabnormality detection unit 2 d may determine based on theseinstructions.

Next, the abnormality detection unit 2 d determines whether the targetposition increasing as time passes is more than or equal to a detectionstart position (“detection start position” in FIG. 3) and is smallerthan or equal to the detection end position (step S102), and branches tothe detection process of the abnormality 1 and the detection process ofthe abnormality 2 based on the determination result.

If the target position is more than or equal to the detection startposition and smaller than or equal to the detection end position (stepS102: Yes), the abnormality detection unit 2 d performs steps S105 toS110 that are included in the detection process of the abnormality 1 (aprocedure enclosed by a dashed line in FIG. 5), and if the targetposition is out of the above-described range (step S102: No), theabnormality detection unit 2 d performs steps S103 and S104 that areincluded in the detection process of the abnormality 2. Since thereexists a dead time immediately after acceleration in the DC motoroperation due to an effect of static friction torque and the like,detection of the abnormality 1 immediately after the start of theacceleration that is likely to be incorrect detection can be avoided bystarting the processing after the time length corresponding to the deadtime. The reason for defining the time period to perform the detectionprocess of the abnormality 1 is to prevent the likelihood of incorrectdetection.

Note that since the target position used for the determination in stepS102 varies its target value as a set value as time passes, theabnormality detection unit 2 d may be configured to use time instead oftarget position in determination in step S102.

If the target position is in the range for performing the detectionprocess of the abnormality 1 (step S102: Yes), a position error of thedetected position calculated by the position detecting unit 9 withrespect to the target position is calculated based on the rotationdetection (encoder) signal (step S105).

Next, the abnormality detection unit 2 d performs the thresholdprocessing for determining whether the position error calculated in stepthe S105 is larger than the second threshold (step S106). Specifically,the abnormality detection unit 2 d determines occurrence of theabnormality with a condition that the calculated position error is morethan the second threshold for determining whether it is a valuecorresponding to occurrence of an abnormality.

An explanation is now added with concrete examples for the method ofdetecting occurrence of an abnormality by the threshold processing withrespect to a position error performed in step S106. FIG. 6 is a graphillustrating the control target position that is a function of time, andposition errors both in abnormal conditions and in normal conditions.

The longitudinal axis in FIG. 6 represents a rotation position of the DCmotor represented by the unit of the detected position [puls] obtainedbased on a rotation detection signal from the encoder 6, and thehorizontal axis represents time [t] that is an elapsed time from thetime point when the control operation is started, which is set as t=0.

FIG. 6 illustrates a position error (error of a position detection valuewith respect to the target value) value as time passes from the controlstart time both in the abnormal conditions 1 and 2 (refer to FIG. 3) andin normal conditions in a control operation of simply increasing atarget position as a control target value (the illustrated example inFIG. 6 is a constant acceleration operation, and the line Lgrepresenting a target position is a line rising increasing to theright).

The line Lc representing a position error value illustrated in FIG. 6represents an error value in normal operation that is an almost constantsmall value after the dead time.

On the other hand, in the conditions of the abnormality 1, the detectedposition value stays at zero and does not vary, and thus the line Le1representing the position error value is almost the same as the targetposition. In the conditions of the abnormality 2, a value of thedetected position simply decreases (displaces in one direction), andthus the line Let representing the position error value rapidlyincreases comparing to the case of the abnormality 1.

Therefore, it is possible to discriminate whether the operation is innormal operating state or an abnormality has occurred by performing thethreshold processing with a threshold that is set slightly larger thanthe error value (line Lc) in normal operation. In addition, regarding anoccurrence detection of the abnormality 1 and the abnormality 2,discrimination is also possible by selecting the threshold.

In the process flow chart of FIG. 5, as a detection process adapted tothe abnormality 1, a threshold appropriate to discriminate a normaloperating state and the abnormality 1 is set in step S106. In addition,as a procedure of the detection process, when it is determined that anabnormality has occurred in the threshold processing of step S106, aloop process for running through a process of step S106 again isrepeated, and with a condition that an occurrence of the abnormalitycontinues for a predetermined time length, it is finally determined thatthe abnormality is an expected abnormality that requires to stopdriving. The second threshold set in step S106 is a value smaller than asecond threshold to be set appropriate to detection of the abnormality 2in step S103 as is described later, and is adapted to both of theabnormality 1 and the abnormality 2.

Therefore, when it is determined that an abnormality has occurredbecause a position error is more than the second threshold in step S106(step S106: Yes), the abnormality detection unit 2 d counts up thetimer, that is, it starts a timer count if a timer is stopped, and if atimer is already started, an operation to continue a timer count of thetimer is performed (step S107).

On the other hand, if it is not determined that an abnormality occursbecause the position error is smaller than or equal to the secondthreshold in step S106 (step S106: No), the abnormality detection unit 2d stops a timer count that is currently timing, and clears the count soas to start a new loop process (step S108).

After the operation of the timer count in step S107 or S108, theabnormality detection unit 2 d confirms whether the current timer countis more than or equal to a predetermined value defined to determine anexpected abnormality that requires to stop driving the DC motor (stepS109).

If the timer count is not more than or equal to the predetermined value(step S109: No), the process returns to the first step S101 of thedetection operation. However, on the way back to step S101, theabnormality detection unit 2 d performs the detection process of theabnormality 2 (step S103) in steps S103 and S104.

On the other hand, if the timer count is more than or equal to thepredetermined value (step S109: Yes), the abnormality detection unit 2 ddetermines that an expected abnormality that requires to stop drivingthe DC motor has occurred (step S110), stops driving the DC motor (stepS111), and ends the abnormality detection process.

In the detection process corresponding to the abnormality 2 of stepS103, the abnormality detection is always performed basically. In thisembodiment, if it is determined that the detection process of theabnormality 1 is not started (step S102: No) since the target positionis out of the set range in step S102 for determining whether to startthe detection process of the abnormality 1 or not by confirming whetherthe target position is in the set range or not, the abnormalitydetection unit 2 d performs the detection process of the abnormality 2(step S103).

In this embodiment, also when the timer count does not reach to thepredetermined value in step S109 (step S109: No), the abnormalitydetection unit 2 d performs the detection process of the abnormality 2(step S103). This is because in the detection process adapted to theabnormality 1, the occurrence of the abnormality 1 that finally requiresto stopping driving is detected with a condition of continuation for apredetermined time length. And on a way of a loop process performed asthe process steps, that is, on a way back from step S109 to S101 inorder to perform the detection process of the abnormality 1 again, thedetecting process of the abnormality 2 is performed.

In the detection process of the abnormality 2 of step S103, theabnormality detection unit 2 d calculates the position error similarlyin steps S105 and S106, and performs the threshold processing withrespect to the calculated position error. However, the second thresholdappropriate for the detection of the abnormality 2 is set to be a largervalue than the second threshold set in step S106 so as to adapt to theposition error of the abnormality 2 that rapidly varies (refer to FIG.6), whereby an abnormality to which the detection process of abnormality1 is to be applied and an abnormality to which the abnormality 2 is tobe applied can be discriminated. Therefore, it is then confirmed whetheran abnormality has been detected in step S103 (step S104), and theprocess branches depending on the detection result of the abnormality 2.

If the abnormality detection is confirmed in step S104 (step S104: Yes),it means that the position error is large making a dangerous situation.Therefore, the abnormality detection unit 2 d determines that anabnormality requiring to stop driving the DC motor has occurred withoutwaiting for the detection result of the abnormality 1, stops driving theDC motor (step S111), and ends the abnormality detection process.

On the other hand, if the abnormality detection is not confirmed in stepS104 (step S104: No), the process returns to the first step S101 of thedetection operation of the abnormality 1.

As described above, by the abnormality detection process of thisembodiment, an abnormality is reliably detected by respectively adaptingto the abnormality 1 and the abnormality 2 of different causes, and thestop control of the DC motor is performed when an abnormality isdetected, whereby a damage on the DC motor driving system (breakage of amechanical system due to runaway thereof, damage due to motor heating,and the like) can be prevented from occurring.

In the abnormality detection process according to the embodimentdescribed above, the abnormality detection process adapted to both ofthe abnormality 1 and the abnormality 2 of different causes is provided.However, a process for detecting either one can be realized by using abasically similar method.

In addition, although the detection method 2 (method based on theposition error) is used in the abnormality detection process accordingto the embodiment described above, the detection method 1 (method basedon the detected position) may be used instead of the method. When thedetection method 1 is used, the threshold processing is different asdescribed above. However, the detection process can be performed with aprocedure similar to the process flow chart of FIG. 5 by replacing stepsS105 and S106, and step S103 in the process flow chart of FIG. 5 withthe threshold processing described above referring to FIG. 3. Such aprocess according to another embodiment is described in detail below.

FIG. 7 is a flow chart illustrating a procedure of the abnormalitydetection process by the detection method 1. Steps S101 and S102 aresimilar to the procedure of the abnormality detection process by thedetection method 1 described referring to FIG. 5. Next, in the detectionmethod 1, the abnormality detection unit 2 d performs the thresholdprocessing for detecting whether the detected position output from theposition detecting unit 9 is smaller than or equal to the firstthreshold (step S306). If the detected position is smaller than or equalto the first threshold (step S306: Yes), the abnormality detection unit2 d counts up a timer (step S307).

On the other hand, if the detected position is more than the firstthreshold in step S306 (step S306: No), the abnormality detection unit 2d clears the timer count (step S308).

Then, the abnormality detection unit 2 d determines whether the timercount is more than or equal to the predetermined value (step S309). Ifthe timer count is more than or equal to the predetermined value (stepS309: Yes), the abnormality detection unit 2 d determines an abnormality(step S310). On the other hand, if the timer count is smaller than thepredetermined value in step S309 (step S309: No), the abnormalitydetection unit 2 d detects the abnormality 2 (step S303). In this step,the detection of the abnormality 2 is performed similarly to thatperformed in step S306. The following process (step S104, S111) will beperformed similarly to the process of the detection method 2.

In the embodiments described above, a position control is described asan example, but a speed control can also be performed using a similardetection method. The speed control may be for an acceleration operationfrom a stopped state (speed: zero), and may also be for an accelerationoperation from a constant speed.

Note that a motor control program that is executed on a computer used asthe controller 2 in the embodiments is previously stored in a ROM or thelike and provided.

The motor control program executed in the computer used as thecontroller 2 according to the embodiments may be configured to beprovided as a file in installable form or executable form that is storedin a computer-readable storage medium such as a CD-ROM, a flexible disk(FD), a CD-R, a DVD (Digital Versatile Disk).

In addition, the motor control program to be executed by the computerused as the controller 2 according to the embodiments may be configuredto be stored in a computer connected a network such as the Internet andto be provided when it is downloaded through the network. Further, themotor control program to be executed on the computer used as thecontroller 2 according to the embodiments may be configured to beprovided or distributed through a network such as the Internet.

The motor control program to be executed on the computer used as thecontroller 2 according to the embodiments has a module configurationincluding each of the above-described units (control unit 2 e,abnormality detection unit 2 d). As an actual hardware, a CPU reads themotor control program from the ROM and executes the program, whereby theeach unit is loaded on a main memory so that the control unit 2 e andthe abnormality detection unit 2 d are generated on the main memory.

The control unit 2 e and the abnormality detection unit 2 d may beconfigured by software as well as hardware.

According to the embodiments, an abnormality occurring upon a drivecontrol for increasing rotation of a motor with time is detected, andthe present invention has an effect that a damage of a motor drivingsystem due to an abnormality such as breakage of a mechanical system dueto runaway thereof, and a damage due to motor heating can be prevented.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A motor control device comprising: an encoderconfigured to detect rotation of a motor to be controlled and output arotation detection signal related to the rotation; a drive controlcircuit configured to perform drive control to rotate the motor at acontrol target value increasing with time based on a rotation detectionvalue calculated from the rotation detection signal; and an abnormalitydetection circuit configured to when a target position of the motor ischanged, begin performing a first abnormality detection process fordetecting a first abnormality related to output of the encoder andperforming a second abnormality detection process for detecting a secondabnormality related to the rotation of the motor, wherein theabnormality detection circuit performs the first abnormality detectionprocess for only a predetermined time period defined either by timeelapsed from start of the drive control or by predetermined controltarget values, continues performance of the second abnormality detectionprocess for a time period following the predetermined time period,performs the first abnormality detection process at least based on therotation detection value and a first threshold, performs the secondabnormality detection process at least based on an error between therotation detection value and a target control value and a secondthreshold, performs the second abnormality detection process bycomparing an error between the control target value and the rotationdetection value with the second threshold until the target positionreaches a first position from start of the drive control, performs thefirst abnormality detection process by comparing the rotation detectionvalue with the first threshold until the target position reaches asecond position from the first position, and performs the secondabnormality detection process by comparing the error with the secondthreshold after the target position reaches the second position, andwherein the drive control circuit performs control to stop rotation ofthe motor when the first abnormality or the second abnormality isdetected.
 2. The motor control device according to claim 1, wherein theabnormality detection circuit detects the first abnormality when a countof how many times the rotation detection value is smaller than the firstthreshold continuously is a predetermined value in the first abnormalitydetection process, and detects the second abnormality when the rotationdetection value is smaller than the second threshold in the secondabnormality detection process.
 3. The motor control device according toclaim 1, wherein the abnormality detection circuit always performs thesecond abnormality detection process after the drive control is started.4. The motor control device according to claim 1, wherein thepredetermined time period is a period to when a target position reachesa predetermined position.
 5. The motor control device according to claim1, wherein the abnormality detection circuit performs the firstabnormality detection process and the second abnormality detectionprocess when the target position increases.
 6. The motor control deviceaccording to claim wherein the abnormality detection circuit performsthe second abnormality detection process after the first abnormalitydetection process in the predetermined time period, and the drivecontrol circuit performs control to stop rotation of the motor withoutperforming the second abnormality detection process when the firstabnormality is detected.
 7. An image formation device, comprising: amotor that conveys a recording medium on which an image is formed; and amotor control device including an encoder configured to detect rotationof the motor to be controlled and output a rotation detection signalrelated to the rotation; a drive control circuit configured to performdrive control to rotate the motor at a control target value increasingwith time based on a rotation detection value calculated from therotation detection signal; and an abnormality detection circuitconfigured to when a target position of the motor is changed, beginperforming a first abnormality detection process for detecting a firstabnormality related to output of the encoder and performing a secondabnormality detection process for detecting a second abnormality relatedto the rotation of the motor, wherein the abnormality detection circuitperforms the first abnormality detection process for only apredetermined time period defined either by time elapsed from start ofthe drive control or by predetermined control target values, continuesperformance of the second abnormality detection process for a timeperiod following the predetermined time period, performs the firstabnormality detection process at least based on the rotation detectionvalue and a first threshold, performs the second abnormality detectionprocess at least based on an error between the rotation detection valueand a target control value and a second threshold, performs the secondabnormality detection process by comparing an error between the controltarget value and the rotation detection value with the second thresholduntil the target position reaches a first position from start of thedrive control, performs the first abnormality detection process bycomparing the rotation detection value with the first threshold untilthe target position reaches a second position from the first position,and performs the second abnormality detection process by comparing theerror with the second threshold after the target position reaches thesecond position, and wherein the drive control circuit performs controlto stop rotation of the motor when the first abnormality or the secondabnormality is detected.
 8. A conveying device, comprising: a motor thatconveys a paper medium to a target position; and a motor control deviceincluding an encoder configured to detect rotation of the motor to becontrolled and output a rotation detection signal related to therotation; a drive control circuit configured to perform drive control torotate the motor at a control target value increasing with time based ona rotation detection value calculated from the rotation detectionsignal; and an abnormality detection circuit configured to when thetarget position of the motor is changed, begin performing a firstabnormality detection process for detecting a first abnormality relatedto output of the encoder and performing a second abnormality detectionprocess for detecting a second abnormality related to the rotation ofthe motor, wherein the abnormality detection circuit performs the firstabnormality detection process for only a predetermined time perioddefined either by time elapsed from start of the drive control or bypredetermined control target values, continues performance of the secondabnormality detection process for a time period following thepredetermined time period, performs the first abnormality detectionprocess at least based on the rotation detection value and a firstthreshold, performs the second abnormality detection process at leastbased on an error between the rotation detection value and a targetcontrol value and a second threshold, performs the second abnormalitydetection process by comparing an error between the control target valueand the rotation detection value with the second threshold until thetarget position reaches a first position from start of the drivecontrol, performs the first abnormality detection process by comparingthe rotation detection value with the first threshold until the targetposition reaches a second position from the first position, and performsthe second abnormality detection process by comparing the error with thesecond threshold after the target position reaches the second position,and wherein the drive control circuit performs control to stop rotationof the motor when the first abnormality or the second abnormality isdetected.
 9. A motor control method comprising: detecting rotation of amotor to be controlled; outputting a rotation detection signal relatedto the rotation; performing drive control to rotate the motor at acontrol target value increasing with time based on a rotation detectionvalue calculated from the rotation detection signal; when a targetposition of the motor is changed, beginning performing a firstabnormality detection process for detecting a first abnormality relatedto output of the encoder and performing a second abnormality detectionprocess for detecting a second abnormality related to the rotation ofthe motor, performing the first abnormality detection process for only apredetermined time period defined either by time elapsed from start ofthe drive control or by predetermined control target values; continuingperformance of the second abnormality detection process for a timeperiod following the predetermined time period; performing the firstabnormality detection process at least based on the rotation detectionvalue and a first threshold; performing the second abnormality detectionprocess at least based on an error between the rotation detection valueand a target control value and a second threshold; performing the secondabnormality detection process by comparing an error between the controltarget value and the rotation detection value with the second thresholduntil the target position reaches a first position from start of thedrive control; performing the first abnormality detection process bycomparing the rotation detection value with the first threshold untilthe target position reaches a second position from the first position;performing the second abnormality detection process by comparing theerror with the second threshold after the target position reaches thesecond position; and performing control to stop rotation of the motorwhen the first abnormality or the second abnormality is detected. 10.The motor control method according to claim 9, further comprising:detecting the first abnormality when a count of how many times therotation detection value is smaller than the first thresholdcontinuously is a predetermined value in the first abnormality detectionprocess; and detecting the second abnormality when the rotationdetection value is smaller than the second threshold in the secondabnormality detection process.
 11. The motor control method according toclaim 9, wherein the second abnormality detection process is alwaysperformed after the drive control is started.
 12. The motor controlmethod according to claim 9, wherein the predetermined time period is aperiod to when a target position reaches a predetermined position. 13.The motor control method according to claim 9, wherein the firstabnormality detection process and the second abnormality detectionprocess are performed when the target position increases.
 14. The motorcontrol method according to claim 9, wherein the second abnormalitydetection process is performed after the first abnormality detectionprocess in the predetermined time period, and the control to stop therotation of the motor is performed without performing the secondabnormality detection process when the first abnormality is detected.